We continue our studies on the structural-functional relationship in hemopoiesis. Using ectopic autoimplants of marrow wherein the marrow stroma is reconstituted before initiation of hemopoiesis. In implants of aplastic, yellow marrow, in rabbits, this phase of hemopoiesis is transient and subsequently undergoes involution. We shall study the involution of hemopoiesis in relation to stromal changes that may precede the disappearance of hemopoiesis. We shall modulate the magnitude of erythropoiesis by hypertransfusion and blood letting. We shall study the stromal changes associated with the modulation of erythropoietic function, in an attempt to relate structure and function. By the use of in vitro erythroid culture (CFU-E and BFU-E), we shall attempt to validate or refute the hypothesis that the development of BFU-E depends on the development of a macrophage population responsible for sustaining erythropoiesis. This is analogous to the central macrophage in erythroblastic islands. Erythroid proliferation is analogous to the central macrophage in erythroblastic islands. Erythroid proliferation in CFU-E is not sustained for the lack of this supporting macrophage system. These observations will focus on membrane relationship between hemopoietic cells and its stroma and in particular the sinusoidal microcirculation of the marrow which is the major component of the marrow-blood barrier. Our observational methods, worked out duing the past few years by our group, include perfusion fixation, application to the marrow of the freeze-fracture (FF) technique, lanthanum impregnation and the use of tannic acid as a mordant to enhance the membrane contrast and delineate membrane relationship in hemopoietic tissues.